Question 213122
Since: Speed=Distance/Time
And Time = Distance/Speed
The distance did not change and we let this value be D
When Speed = 60, Time is T, as in:
...
{{{T = D/60}}}
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When Speed = 70, Time is T-30 minutes(30 minutes is half an hour, speed is expressed as km/h) so, T-0.5, as in:
{{{T-0.5 = D/70 }}}
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Substituting the value of T from the first equation, we get:
{{{(D/60)-0.5 = D/70}}}
{{{(D/60)-(D/70) = 0.5}}}
Getting the LCD, 420 we get:
{{{(7D-6D)/420=0.5}}}
Multiplying both sides by 420 we get:
{{{D = 0.5*420 = 210 km}}}
So the distance involved is 210km